Low cost strip chart recorders are often used to record the temperature of a cargo during transport. Such chart recorders must be constructed at very low cost, and generally use pressure-markable chart paper. A common type of pressure-markable chart paper includes a base of black color and a film of white opaque wax on the base, the wax being readily removable by the point of a stylus. Another type of pressure-markable paper includes a layer of microscopic ink-filled capsules that are broken by a stylus to record a trace.
Previously, the temperature sensor for the strip chart recorder was attached to the recorder housing to measure the temperature at the recorder. However, there are many instances where two or more different temperatures of other environmental conditions must be measured, such as the temperatures at inside walls of a truck or other large container rather than just at the chart recorder that may lie in the middle of the container or at only one wall. If a single low-cost chart recorder with a narrow strip of chart paper is to be used, then the traces representing the two measurements will both be capable of lying anywhere across substantially the entire width of the same strip of chart paper. This can cause the pressure-marked traces to sometimes cross each other, making it difficult to distinguish one trace representing the temperature at one location, from another trace representing the temperature at another location. Apparatus which enabled two or more different styluses to be moved independently across most of the width of a strip of chart paper without interference from each other, and which allowed each trace to be readily distinguished from one or more other traces, and which still enabled the chart recorder to be constructed at low cost, would be of considerable value.